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Cellular activity in neocortical circuits and behavior

The main interest of my lab is to understand how the properties of neocortical neurons, the circuits they form and the inputs they receive give rise to neuronal activity and behavior. Our approach includes behavioral studies, two-photon calcium imaging, in vivo whole cell recording in behaving animals and optogenetic methods to modulate activity of cortical neurons.

The neocortex represents about 80% of the human brain and is associated with wide range of functions including sensory perception, motor movement, memory and higher aspects of cognition. In the neocortex a major synaptic input originates from local cortical neurons. In addition, thalamic inputs, together with distant corticortical inputs and neuromodulator inputs play critical role. How these diverse inputs combine at the cellular and circuit level to generate neuronal activity and behavior is the focus of our studies.

We have developed methods to define cell types using genetics and other means. In vitro approach allowed us to characterize the wiring pattern of microcircuits and the synaptic properties of specific connections. To investigate how neuronal properties and interactions generate neuronal activity under physiological conditions we study cortical neurons in vivo in behaving mice. By combining whole cell recordings, calcium imaging and optogenetics we study the subthreshold activity and circuit activity that drive spike generation in cortical neurons. Importantly, by studying and manipulating neuronal responses as well as behavioral performance we can investigate the relation between neuronal activity and animal perception.

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Abstract

Neurons in mouse V1 increase their response to visual stimulation during locomotion. In this issue of Neuron, Lee et al. (2014) show that subthreshold optogenetic stimulation of a brainstem locomotion area can mimic the effect of locomotion on sensory processing.

Abstract

Layer 6 corticothalamic neurons are thought to modulate incoming sensory information via their intracortical axons targeting the major thalamorecipient layer of the neocortex, layer 4, and via their long-range feedback projections to primary sensory thalamic nuclei. However, anatomical reconstructions of individual layer 6 corticothalamic (L6 CT) neurons include examples with axonal processes ramifying within layer 5, and the relative input of the overall population of L6 CT neurons to layers 4 and 5 is not well understood. We compared the synaptic impact of L6 CT cells on neurons in layers 4 and 5. We found that the axons of L6 CT neurons densely ramified within layer 5a in both visual and somatosensory cortices of the mouse. Optogenetic activation of corticothalamic neurons generated large EPSPs in pyramidal neurons in layer 5a. In contrast, excitatory neurons in layer 4 exhibited weak excitation or disynaptic inhibition. Fast-spiking parvalbumin-positive cells in both layer 5a and layer 4 were also strongly activated by L6 CT neurons. The overall effect of L6 CT activation was to suppress layer 4 while eliciting action potentials in layer 5a pyramidal neurons. Together, our data indicate that L6 CT neurons strongly activate an output layer of the cortex.

Abstract

The ascending cholinergic neuromodulatory system sends projections throughout cortex and has been shown to play an important role in a number of cognitive functions including arousal, working memory, and attention. However, despite a wealth of behavioral and anatomical data, understanding how cholinergic synapses modulate cortical function has been limited by the inability to selectively activate cholinergic axons. Now, with the development of optogenetic tools and cell-type specific Cre-driver mouse lines, it has become possible to stimulate cholinergic axons from the basal forebrain (BF) and probe cholinergic synapses in the cortex for the first time. Here we review recent work studying the cell-type specificity of nicotinic signaling in the cortex, synaptic mechanisms mediating cholinergic transmission, and the potential functional role of nicotinic modulation.

Abstract

The processing of sensory information varies widely across behavioral states. However, little is known about how behavioral states modulate the intracellular activity of cortical neurons to effect changes in sensory responses. Here, we performed whole-cell recordings from neurons in upper-layer primary visual cortex of awake mice during locomotion and quiet wakefulness. We found that the signal-to-noise ratio for sensory responses was improved during locomotion by two mechanisms: (1) a decrease in membrane potential variability leading to a reduction in background firing rates and (2) an enhancement in the amplitude and reliability of visually evoked subthreshold responses mediated by an increase in total conductance and a depolarization of the stimulus-evoked reversal potential. Consistent with the enhanced signal-to-noise ratio for visual responses during locomotion, we demonstrate that performance is improved in a visual detection task during this behavioral state.

Abstract

A systematic classification and accepted nomenclature of neuron types is much needed but is currently lacking. This article describes a possible taxonomical solution for classifying GABAergic interneurons of the cerebral cortex based on a novel, web-based interactive system that allows experts to classify neurons with pre-determined criteria. Using Bayesian analysis and clustering algorithms on the resulting data, we investigated the suitability of several anatomical terms and neuron names for cortical GABAergic interneurons. Moreover, we show that supervised classification models could automatically categorize interneurons in agreement with experts' assignments. These results demonstrate a practical and objective approach to the naming, characterization and classification of neurons based on community consensus.

Abstract

Activation of cortical nicotinic receptors by cholinergic axons from the basal forebrain (BF) significantly impacts cortical function, and the loss of nicotinic receptors is a hallmark of aging and neurodegenerative disease. We have previously shown that stimulation of BF axons generates a fast ?7 and a slow non-?7 receptor-dependent response in cortical interneurons. However, the synaptic mechanisms that underlie this dual-component nicotinic response remain unclear. Here, we report that fast ?7 receptor-mediated EPSCs in the mouse cortex are highly variable and insensitive to perturbations of acetylcholinesterase (AChE), while slow non-?7 receptor-mediated EPSCs are reliable and highly sensitive to AChE activity. Based on these data, we propose that the fast and slow nicotinic responses reflect differences in synaptic structure between cholinergic varicosities activating ?7 and non-?7 classes of nicotinic receptors.

Abstract

Cholinergic activation of nicotinic receptors in the cortex plays a critical role in arousal, attention, and learning. Here we demonstrate that cholinergic axons from the basal forebrain of mice excite a specific subset of cortical interneurons via a remarkably slow, non-?7 nicotinic receptor-mediated conductance. In turn, these inhibitory cells generate a delayed and prolonged wave of disynaptic inhibition in neighboring cortical neurons, altering the spatiotemporal pattern of inhibition in cortical circuits.

Abstract

What are the mechanisms that enhance the response to behaviorally relevant external stimuli? In this issue of Neuron, Kuo and Trussell show that in the dorsal cochlear nucleus, noradrenaline functions to simultaneously reduce spontaneous inhibitory inputs while increasing evoked inhibition.

Abstract

Parvalbumin-expressing fast-spiking (FS) cells are interconnected via GABAergic and electrical synapses and represent a major class of inhibitory interneurons in the neocortex. Synaptic connections among FS cells are critical for regulating network oscillations in the mature neocortex. However, it is unclear whether synaptic connections among FS interneurons also play a central role in the generation of patterned neuronal activity in the immature brain, which is thought to underlie the formation of neocortical circuits. Here, we investigated the developmental time course of synaptogenesis of FS cell in mouse visual cortex. In layer 5/6 (L5/6), we recorded from two or three FS and/or pyramidal (PYR) neurons to study the development of electrical and chemical synaptic interactions from postnatal day 3 (P3) to P18. We detected no evidence for functional connectivity for FS-FS or FS-PYR pairs at P3-P4. However, by P5-P6, we found that 20% of FS pairs were electrically coupled, and 24% of pairs were connected via GABAergic synapses; by P15-P18, 42% of FS pairs had established functional electrical synapses, and 47% of FS pairs were connected via GABAergic synapses. FS cell GABAergic inhibition of pyramidal cells showed a similar developmental time line, but no electrical coupling was detected for FS-PYR pairs. We found that synaptogenesis of electrical and GABAergic connections of FS cells takes place in the same period. Together, our results suggest that chemical and electrical connections among FS cells can contribute to patterned neocortical activity only by the end of the first postnatal week.

Abstract

A central tenet of neuroscience is that the precise patterns of connectivity among neurons in a given brain area underlie its function. However, assigning any aspect of perception or behavior to the wiring of local circuits has been challenging. Here, we review recent work in sensory neocortex that demonstrates the power of identifying specific cell types when investigating the functional organization of brain circuits. These studies indicate that knowing the identity of both the presynaptic and postsynaptic cell type is key when analyzing neocortical circuits. Furthermore, identifying the circuit organization of particular cell types in the neocortex allows the recording and manipulation of each cell type's activity and the direct testing of its functional role in perception and behavior.

Abstract

Cortical columns generate separate streams of information that are distributed to numerous cortical and subcortical brain regions. We asked whether local intracortical circuits reflect these different processing streams by testing whether the intracortical connectivity among pyramidal neurons reflects their long-range axonal targets. We recorded simultaneously from up to four retrogradely labelled pyramidal neurons that projected to the superior colliculus, the contralateral striatum or the contralateral cortex to assess their synaptic connectivity. Here we show that the probability of synaptic connection depends on the functional identities of both the presynaptic and postsynaptic neurons. We first found that the frequency of monosynaptic connections among corticostriatal pyramidal neurons is significantly higher than among corticocortical or corticotectal pyramidal neurons. We then show that the probability of feed-forward connections from corticocortical neurons to corticotectal neurons is approximately three- to fourfold higher than the probability of monosynaptic connections among corticocortical or corticotectal cells. Moreover, we found that the average axodendritic overlap of the presynaptic and postsynaptic pyramidal neurons could not fully explain the differences in connection probability that we observed. The selective synaptic interactions we describe demonstrate that the organization of local networks of pyramidal cells reflects the long-range targets of both the presynaptic and postsynaptic neurons.

Abstract

Distinct networks of gamma-aminobutyric acidergic interneurons connected by electrical synapses can promote different patterns of activity in the neocortex. Cannabinoids affect memory and cognition by powerfully modulating a subset of inhibitory synapses; however, very little is known about the synaptic properties of the cannabinoid receptor-expressing neurons (CB(1)-positive irregular spiking [CB(1)-IS]) in the neocortex. Using paired recordings in neocortical slices, we 1st report here that synapses of CB(1)-IS cells, but not synapses of fast-spiking (FS) cells, are suppressed by release of endocannabinoids from pyramidal neurons. CB(1)-IS synapses were characterized by a very high failure rate that contrasted with the high reliability of FS synapses. Furthermore, CB(1)-IS cells received excitatory inputs less frequently compared with FS cells and made significantly less frequent inhibitory contacts onto local pyramids. These distinct synaptic properties together with their characteristic irregular firing suggest that CB(1)-IS cells play different role in neocortical function than that of FS cells. Thus, whereas the synaptic properties of FS cells can allow them to impose high-frequency rhythmic oscillatory activity, those of CB(1)-IS cells may lead to disruption of fast rhythmic oscillations. Our results suggest that activity-dependent release of cannabinoids, by blocking CB(1)-IS synapses, may alter the role of inhibition in neocortical circuits.

Abstract

Neuroscience produces a vast amount of data from an enormous diversity of neurons. A neuronal classification system is essential to organize such data and the knowledge that is derived from them. Classification depends on the unequivocal identification of the features that distinguish one type of neuron from another. The problems inherent in this are particularly acute when studying cortical interneurons. To tackle this, we convened a representative group of researchers to agree on a set of terms to describe the anatomical, physiological and molecular features of GABAergic interneurons of the cerebral cortex. The resulting terminology might provide a stepping stone towards a future classification of these complex and heterogeneous cells. Consistent adoption will be important for the success of such an initiative, and we also encourage the active involvement of the broader scientific community in the dynamic evolution of this project.

Abstract

Dopamine, acting through D(1) receptors, is thought to play an important role in cognitive functions of the frontal cortex such as working memory. D(1) receptors are widely expressed in fast-spiking (FS) interneurons, a prominent class of inhibitory cells that exert a powerful control of neuronal firing through proximal synapses on their postsynaptic targets. FS cells are extensively mutually interconnected by both GABA(A) receptor-mediated synapses and gap junction-mediated electrical synapses, and networks of FS cells play a crucial role in the generation of rhythmic synchronous activity. Although recent studies have documented the effects of dopamine modulation of neocortical synaptic connections among excitatory cells and between excitatory and various inhibitory cells, the effects of dopamine receptor activation on GABAergic and electrical interactions among FS cells is not known. To resolve this, we recorded from pairs of FS cells in the infragranular layers of mouse neocortical slices and tested the effects of D(1)-like (D(1)/D(5)) receptor activation on these connections. We found that D(1)-like receptor activation modulated GABAergic but not electrical connections between them. A D(1)-like receptor agonist preserved the strength of electrical coupling but reduced the amplitude of IPSPs and IPSCs between FS cells. Our results suggest that D(1)-like receptor activation has synapse-specific effects within networks of FS cells, with potential implications for the generation of rhythmic activity in the neocortex.

Abstract

The temporal precision of converting excitatory postsynaptic potentials (EPSPs) into spikes at pyramidal cells is critical for the coding of information in the cortex. Several in vitro studies have shown that voltage-dependent conductances in pyramidal cells can prolong the EPSP time course resulting in an imprecise EPSP-spike coupling. We have used dynamic-clamp techniques to mimic the in vivo background synaptic conductance in cortical slices and investigated how the ongoing synaptic activity may affect the EPSP time course near threshold and the EPSP spike coupling. We report here that background synaptic conductance dramatically diminished the depolarization related prolongation of the EPSPs in pyramidal cells and improved the precision of spike timing. Furthermore, we found that background synaptic conductance can affect the interaction among action potentials in a spike train. Thus the level of ongoing synaptic activity in the cortex may regulate the capacity of pyramidal cells to process temporal information.

Abstract

Anatomical studies have shown that the G-protein-coupled cannabinoid receptor-1 (CB1) is selectively expressed in a subset of GABAergic interneurons. It has been proposed that these cells regulate rhythmic activity and play a key role mediating the cognitive actions of marijuana and endogenous cannabinoids. However, the physiology, anatomy, and synaptic connectivity of neocortical CB1-expressing interneurons remain poorly studied. We identified a population of CB1-expressing interneurons in layer II/III in mouse neocortical slices. These cells were multipolar or bitufted, had a widely extending axon, and exhibited a characteristic pattern of irregular spiking (IS) in response to current injection. CB1-expressing-IS (CB1-IS) cells were inhibitory, establishing GABAA receptor-mediated synapses onto pyramidal cells and other CB1-IS cells. Recently, electrical coupling among other classes of cortical interneurons has been shown to contribute to the generation of rhythmic synchronous activity in the neocortex. We therefore tested whether CB1-IS interneurons are interconnected via electrical synapses using paired recordings. We found that 90% (19 of 21 pairs) of simultaneously recorded pairs of CB1-IS cells were electrically coupled. The average coupling coefficient was 6%. Signaling through electrical synapses promoted coordinated firing among CB1-IS cells. Together, our results identify a population of electrically coupled CB1-IS GABAergic interneurons in the neocortex that share a unique morphology and a characteristic pattern of irregular spiking in response to current injection. The synaptic interactions of these cells may play an important role mediating the cognitive actions of cannabinoids and regulating coherent neocortical activity.

Abstract

Layer 1 of the neocortex is an important zone in which synaptic integration of inputs originating from a variety of cerebral regions is thought to take place. Layer 1 does not contain pyramidal cells, and several histochemical studies have suggested that most layer 1 neurons are GABAergic. However, although layer 1 neurons could be an important source of inhibition in this layer, the synaptic action of these neurons and the identity of their postsynaptic targets are unknown. We studied the physiological properties and synaptic interactions of a class of cells within layer 1 called late-spiking (LS) cells. The dendrites and axons of layer 1 LS cells were confined primarily to layer 1. Using paired recording, we showed that LS cells formed GABAergic connections with other LS cells as well as with non-LS cells in layer 1 and with pyramidal cells in layer 2/3. We also found that layer 2/3 pyramidal neurons provide excitatory inputs to LS cells. It has been suggested previously that GABAergic neurons belonging to the same class in the cortex are electrically coupled. In agreement with that hypothesis, we found that LS cells were interconnected by electrical coupling (83%), whereas electrical coupling between LS cells and non-LS cells was infrequent (2%). Thus, we provide evidence showing that a group of GABAergic neurons within layer 1 are specifically interconnected by electrical coupling and can provide significant inhibitory inputs to neurons in layer 1 and to distal dendrites of pyramidal cells.

Electrical and chemical synapses among parvalbumin fast-spiking GABAergic interneurons in adult mouse neocortexPROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICAGalarreta, M., Hestrin, S.2002; 99 (19): 12438-12443

Abstract

Networks of gamma-aminobutyric acid (GABA)ergic interneurons connected via electrical and chemical synapses are thought to play an important role in detecting and promoting synchronous activity in the cerebral cortex. Although the properties of electrical and chemical synaptic interactions among inhibitory interneurons are critical for their function as a network, they have only been studied systematically in juvenile animals. Here, we have used transgenic mice expressing the enhanced green fluorescent protein in cells containing parvalbumin (PV) to study the synaptic connectivity among fast-spiking (FS) cells in slices from adult animals (2-7 months old). We have recorded from pairs of PV-FS cells and found that the majority of them were electrically coupled (61%, 14 of 23 pairs). In addition, 78% of the pairs were connected via GABAergic chemical synapses, often reciprocally. The average coupling coefficient for step injections was 1.5% (n = 14), a smaller value than that reported in juvenile animals. GABA-mediated inhibitory postsynaptic currents and potentials decayed with exponential time constants of 2.6 and 5.9 ms, respectively, and exhibited paired-pulse depression (50-ms interval). The inhibitory synaptic responses in the adult were faster than those observed in young animals. Our results indicate that PV-FS cells are highly interconnected in the adult cerebral cortex by both electrical and chemical synapses, establishing networks that can have important implications for coordinating activity in cortical circuits.

Abstract

The temporal pattern and relative timing of action potentials among neocortical neurons may carry important information. However, how cortical circuits detect or generate coherent activity remains unclear. Using paired recordings in rat neocortical slices, we found that the firing of fast-spiking cells can reflect the spiking pattern of single-axon pyramidal inputs. Moreover, this property allowed groups of fast-spiking cells interconnected by electrical and gamma-aminobutyric acid (GABA)-releasing (GABAergic) synapses to detect the relative timing of their excitatory inputs. These results indicate that networks of fast-spiking cells may play a role in the detection and promotion of synchronous activity within the neocortex.

Abstract

Although gap junctions were first demonstrated in the mammalian brain about 30 years ago, the distribution and role of electrical synapses have remained elusive. A series of recent reports has demonstrated that inhibitory interneurons in the cerebral cortex, thalamus, striatum and cerebellum are extensively interconnected by electrical synapses. Investigators have used paired recordings to reveal directly the presence of electrical synapses among identified cell types. These studies indicate that electrical coupling is a fundamental feature of local inhibitory circuits and suggest that electrical synapses define functionally diverse networks of GABA-releasing interneurons. Here, we discuss these results, their possible functional significance and the insights into neuronal circuit organization that have emerged from them.

Abstract

The increased release of oxytocin during lactation has been shown to be dependent upon glutamatergic transmission and is associated with an increased synaptic innervation of the supraoptic nucleus (SON). To determine whether the glutamatergic synaptic properties of oxytocin neurones are changed during lactation, we recorded excitatory postsynaptic currents (EPSCs) from identified oxytocin neurones in the SON of slices taken from adult virgin and lactating rats. The frequency of AMPA-mediated miniature EPSCs (mEPSCs) more than doubled during lactation. In addition, the decay time constant, but not the amplitude of the mEPSCs was significantly increased in both vasopressin and oxytocin neurones. Paired-pulse facilitation (PPF) was significantly reduced in oxytocin neurones during lactation, whereas no change was observed in vasopressin neurones. Elevating Ca(2+) reduced PPF in oxytocin neurones in virgin rats but did not alter PPF in oxytocin neurones from lactating rats. Collectively, our results suggest that excitatory glutamatergic transmission is strengthened in oxytocin neurones during lactation, probably by a combination of an increased number of terminals, slower decay kinetics, and an increase in the probability of release.

Abstract

High-frequency activity produces transient depression at many synapses but also, as recently demonstrated, may accelerate the recovery from use-dependent depression. We have examined the possible consequences of this synaptic mechanism in neocortical excitatory synapses by recording simultaneously from presynaptic pyramidal neurons and their postsynaptic targets. Brief bursts of high-frequency spikes produced a strong depression of the amplitude of unitary excitatory postsynaptic currents (uEPSCs). However, when burst firing was combined with low-frequency ongoing activity, we found that the strong synaptic depression was followed by a transient rebound of synaptic strength. This rebound overshot the low-frequency baseline values and lasted 1-2 s. These results suggest that in the presence of ongoing activity, neocortical synapses may functionally facilitate following burst firing.

Abstract

Encoding of information in the cortex is thought to depend on synchronous firing of cortical neurons. Inhibitory neurons are known to be critical in the coordination of cortical activity, but how interaction among inhibitory cells promotes synchrony is not well understood. To address this issue directly, we have recorded simultaneously from pairs of fast-spiking (FS) cells, a type of gamma-aminobutyric acid (GABA)-containing neocortical interneuron. Here we report a high occurrence of electrical coupling among FS cells. Electrical synapses were not found among pyramidal neurons or between FS cells and other cortical cells. Some FS cells were interconnected by both electrical and GABAergic synapses. We show that communication through electrical synapses allows excitatory signalling among inhibitory cells and promotes their synchronous spiking. These results indicate that electrical synapses establish a network of fast-spiking cells in the neocortex which may play a key role in coordinating cortical activity.

Abstract

Oxytocin (OT) and vasopressin (VP) hormone release from neurohypophysial terminals is controlled by the firing pattern of neurosecretory cells located in the hypothalamic supraoptic (SON) and paraventricular nuclei. Although glutamate is a key modulator of the electrical activity of both OT and VP neurons, a differential contribution of AMPA receptors (AMPARs) and NMDA receptors (NMDARs) has been proposed to mediate glutamatergic influences on these neurons. In the present study we examined the distribution and functional properties of synaptic currents mediated by AMPARs and NMDARs in immunoidentified SON neurons. Our results suggest that the properties of AMPA-mediated currents in SON neurons are controlled in a cell type-specific manner. OT neurons displayed AMPA-mediated miniature EPSCs (mEPSCs) with larger amplitude and faster decay kinetics than VP neurons. Furthermore, a peak-scaled nonstationary noise analysis of mEPSCs revealed a larger estimated single-channel conductance of AMPARs expressed in OT neurons. High-frequency summation of AMPA-mediated excitatory postsynaptic potentials was smaller in OT neurons. In both cell types, AMPA-mediated synaptic currents showed inward rectification, which was more pronounced in OT neurons, and displayed Ca2+ permeability. On the other hand, NMDA-mediated mEPSCs of both cell types had similar amplitude and kinetic properties. The cell type-specific expression of functionally different AMPARs can contribute to the adoption of different firing patterns by these neuroendocrine neurons in response to physiological stimuli.

Frequency-dependent synaptic depression and the balance of excitation and inhibition in the neocortexNATURE NEUROSCIENCEGalarreta, M., Hestrin, S.1998; 1 (7): 587-594

Abstract

The stability of cortical neuron activity in vivo suggests that the firing rates of both excitatory and inhibitory neurons are dynamically adjusted. Using dual recordings from excitatory pyramidal neurons and inhibitory fast-spiking neurons in neocortical slices, we report that sustained activation by trains of several hundred presynaptic spikes resulted in much stronger depression of synaptic currents at excitatory synapses than at inhibitory ones. The steady-state synaptic depression was frequency dependent and reflected presynaptic function. These results suggest that inhibitory terminals of fast-spiking cells are better equipped to support prolonged transmitter release at a high frequency compared with excitatory ones. This difference in frequency-dependent depression could produce a relative increase in the impact of inhibition during periods of high global activity and promote the stability of cortical circuits.

Abstract

We used paired recordings to study the development of synaptic transmission between inhibitory interneurons of the molecular layer and Purkinje cells in the cerebellar cortex of the rat. The electrophysiological data were combined with a morphological study of the recorded cells using biocytin or Lucifer yellow staining. Thirty-one interneuron-Purkinje cell pairs were obtained, and 11 of them were recovered morphologically. The age of the rats ranged from 11 to 31 d after birth. During this period synaptic maturation resulted in an 11-fold decrease in the average current evoked in a Purkinje cell by a spike in a presynaptic interneuron. Unitary IPSCs in younger animals exhibited paired-pulse depression, whereas paired-pulse facilitation was found in more mature animals. These data suggest that reduction in transmitter release probability contributed to the developmental decrease of unitary IPSCs. However, additional mechanisms at both presynaptic and postsynaptic loci should also be considered. The decrease of the average synaptic current evoked in a Purkinje cell by an action potential in a single interneuron suggests that as development proceeds interneuron activities must be coordinated to inhibit efficiently Purkinje cells.

Abstract

Rapid applications of GABA (from 10 microM to 10 mM) to outside-out patches were used to study the role that the kinetic properties of GABAA receptors play in determining the time course of IPSCs in neocortical pyramidal neurons. Currents induced by rapid applications of brief (1 msec) pulses of GABA (1 mM) showed a biexponential decay phase that seems to involve the entry of GABAA receptors into desensitized states. This conclusion is based on the similar fast decay kinetics of the response to brief and prolonged pulses of GABA and on the correlation between the degree of paired-pulse depression and the decay rate of the currents induced by brief pulses. Under nonequilibrium conditions we found that the concentration-response curve of pyramidal GABAA receptors has an EC50 of 185 microM (GABA pulse of 1 msec). The decay time course of the patch currents in response to brief applications of GABA was insensitive to agonist concentrations at the range from 50 microM to 10 mM. Faster decay rates were observed only in response to pulses of 10 microM GABA. These data are compatible with the suggestion that briefer openings derive from a monoliganded state and that these are negligible when receptor activation is >2%. Assuming that GABA transients at neocortical synapses are fast, a several millimolar GABA concentration would be needed to saturate the postsynaptic GABAA receptors.

Abstract

Native AMPA receptors (AMPARs) were investigated in neocortical fast-spiking (FS) and regular-spiking nonpyramidal (RSNP) cells. The onset of and recovery from desensitization as well as current rectification and single-channel conductance were studied by using fast glutamate application to outside-out patches. The GluR1-4 subunit, flip/flop splicing, and R/G editing expression patterns of functionally characterized cells were determined by single-cell reverse transcription-PCR to correlate the subunit composition of native AMPARs with their functional properties. Our sample, mostly constituted by RSNP neurons, predominantly expressed GluR3 flip and GluR2 flop. In individual cells, flip/flop splicing of each subunit appeared to be regulated independently, whereas for R/G editing all subunits were either almost fully edited or unedited. We confirmed that the relative GluR2 expression controls the permeation properties of native AMPARs, whereas none of the single molecular parameters considered appeared to be a key determinant of the kinetics. FS neurons displayed AMPARs with relatively homogeneous functional properties characterized by fast desensitization, slow recovery from desensitization, marked inward rectification, and large single-channel conductance. In contrast, these parameters varied over a wide range in RSNP neurons, and their combination resulted in various AMPAR functional patterns. Indeed, in different cells, fast or slow desensitization was found to be associated with either slow or fast recovery from desensitization. Similarly, fast or slow kinetics was associated with either strong or weak rectification. Our results suggest that kinetic and permeation properties of native AMPARs can be regulated independently in cortical neurons and probably do not have the same molecular determinants.

Abstract

The physiological and molecular features of nonpyramidal cells were investigated in acute slices of sensory-motor cortex using whole-cell recordings combined with single-cell RT-PCR to detect simultaneously the mRNAs of three calcium binding proteins (calbindin D28k, parvalbumin, and calretinin) and four neuropeptides (neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, and cholecystokinin). In the 97 neurons analyzed, all expressed mRNAs of at least one calcium binding protein, and the majority (n = 73) contained mRNAs of at least one neuropeptide. Three groups of nonpyramidal cells were defined according to their firing pattern. (1) Fast spiking cells (n = 34) displayed tonic discharges of fast action potentials with no accommodation. They expressed parvalbumin (n = 30) and/or calbindin (n = 19) mRNAs, and half of them also contained transcripts of at least one of the four neuropeptides. (2) Regular spiking nonpyramidal cells (n = 48) displayed a firing behavior characterized by a marked accommodation and presented a large diversity of expression patterns of the seven biochemical markers. (3) Finally, a small population of vertically oriented bipolar cells, termed irregular spiking cells (n = 15), fired bursts of action potentials at an irregular frequency. They consistently co-expressed calretinin and vasoactive intestinal polypeptide. Additional investigations of these cells showed that they also co-expressed glutamic acid decarboxylase and choline acetyl transferase. Our results indicate that neocortical nonpyramidal neurons display a large diversity in their firing properties and biochemical patterns of co-expression and that both characteristics could be correlated to define discrete subpopulations.

Abstract

The electrophysiological and morphological properties of layer I neurons were studied in visual cortex slices from 7- to 19-d-old rats using whole-cell recording and biocytin labeling. A heterogeneous population of small, nonpyramidal neurons was found. Approximately one third of the cells we recorded were neurogliaform cells; another third were multipolar neurons with axons descending out of layer I. The remaining cells were heterogeneous and were not classified. In slices from 7- to 10-d-old animals only, we identified Cajal-Retzius cells. Neurogliaform neurons had a very dense local axonal field, which was largely contained within layer I. Cells with descending axons had a relatively sparse local axonal arbor and projected at least to layer II and sometimes deeper. Spiking in neurogliaform neurons was followed by an afterdepolarizing potential, whereas spiking in cells with descending axons was followed by a slow after-hyperpolarizing potential (AHP). In addition, neurogliaform cells exhibited less spike broadening and a larger fast AHP after single spikes than did cells with descending axons. Generally, cells in layer I received synaptic inputs characterized as either GABA- or glutamate-mediated, suggesting the presence of excitatory and inhibitory inputs. With their output largely limited to layer I, neurogliaform cells could synapse with other layer I neurons, the most distal dendritic branches of pyramidal cells, or the dendrites of layer II/III interneurons, which invade layer I. Cells with descending axons could contact a wide variety of cortical cells throughout their vertical projection.

Correlation between kinetics and RNA splicing of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in neocortical neuronsPROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICALambolez, B., Ropert, N., Perrais, D., Rossier, J., Hestrin, S.1996; 93 (5): 1797-1802

Abstract

In the cortex fast excitatory synaptic currents onto excitatory pyramidal neurons and inhibitory nonpyramidal neurons are mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors exhibiting cell-type-specific differences in their kinetic properties. AMPA receptors consist of four subunits (GluR1-4), each existing as two splice variants, flip and flop, which critically affect the desensitization properties of receptors expressed in heterologous systems. Using single cell reverse transcription PCR to analyze the mRNA of AMPA receptor subunits expressed in layers I-III neocortical neurons, we find that 90% of the GluR1-4 in nonpyramidal neurons are flop variants, whereas 92% of the GluR1-4 in pyramidal neurons are flip variants. We also find that nonpyramidal neurons predominantly express GluR1 mRNA (GluR1/GluR1-4 = 59%), whereas pyramidal neurons contain mainly GluR2 mRNA (GluR2/GluR1-4 = 59%). However, the neuron-type-specific splicing is exhibited by all four AMPA receptor subunits. We suggest that the predominance of the flop variants contributes to the faster and more extensive desensitization in nonpyramidal neurons, compared to pyramidal cells where flip variants are dominant. Alternative splicing of AMPA receptors may play an important role in regulating synaptic function in a cell-type-specific manner, without changing permeation properties.

Abstract

The biochemical and functional characteristics of the AMPA subtype of the glutamate receptors expressed by pyramidal and non-pyramidal neurons of the neocortex have been studied in acute slices by means of single-cell RT-PCR and fast applications of glutamate on outside-out patches. Our results suggest that the predominant expression of the flop splice variants of the GluR1-4 AMPA subunits contributes to the faster desensitization of these receptors in non-pyramidal neurons compared to pyramidal cells where flip variants of GluR1-4 are dominant. Alternative splicing of AMPA receptors may therefore play an important role in regulating synaptic function in a cell-type specific manner.

Abstract

Spontaneous excitatory postsynaptic currents (sEPSCs) and responses to rapid application of glutamate were recorded in excitatory spiny, pyramidal neurons and compared with those recorded in inhibitory aspiny interneurons. The sEPSC decay time constant was faster in aspiny interneurons (2.5 ms) compared with pyramidal neurons (4.6 ms). The decay time constant in response to a brief application (1 ms) of glutamate (10 mM) in patches excised from pyramidal and aspiny interneurons were similar (1.9 and 2.7 ms, respectively). However, the rate of desensitization was faster in patches from interneurons compared with pyramidal neurons (3.4 and 12.0 ms, respectively). In addition, single-channel conductance was larger in aspiny interneurons (27 pS) compared with pyramidal neurons (9 pS). These results indicate that pyramidal neurons and aspiny interneurons express different non-N-methyl-D-aspartate receptors and that selective desensitization of interneuron receptors may contribute to depression of inhibition.

Abstract

Brief glutamate applications to membrane patches, excised from neurons in the rat visual cortex, were used to assess the role of desensitization in determining the AMPA/kainate receptor-mediated excitatory postsynaptic current (EPSC) time course. A brief (1 ms) application of glutamate (1-10 mM) produced a response that mimicked the time course of miniature EPSCs (mEPSCs). Direct evidence is presented that the rate of onset of desensitization is much slower than the decay rate of the response to a brief application of glutamate, implying that the decay of mEPSCs reflects channel closure into a state readily available for reactivation. Rapid application of glutamate combined with nonstationary variance analysis provided an estimate of the single-channel conductance and open probability, allowing an approximation of the number of available channels at a single synaptic site.

Abstract

The central nervous system has extraordinary plasticity in early life. This is thought to involve N-methyl-D-aspartate (NMDA) receptors which, along with the non-NMDA receptors, mediate fast excitatory synaptic transmission. Although NMDA receptors may be transiently enhanced early in life, it has not been possible to demonstrate directly a functional change in the NMDA receptor-mediated synaptic response because of the voltage-dependence of the NMDA conductance and the overlapping inhibitory synaptic conductances. Here I report that the duration of evoked NMDA-receptor-mediated excitatory postsynaptic currents (e.p.s.cs) in the superior colliculus is several times longer at early developmental stages compared to that measured in older animals. In contrast, the amplitude of NMDA-receptor-mediated miniature e.p.s.cs does not change during development. The kinetic response of excised membrane patches to a brief activation of NMDA receptors is similar to that of the NMDA e.p.s.c, which suggests that the time course of the NMDA e.p.s.c. in the superior colliculus reflects slow NMDA channel properties as in the hippocampus. Therefore, these data indicate that the molecular properties of NMDA receptors are developmentally regulated and thus may be controlling the ability of synapses to change in early life.

Abstract

1. We studied excitatory synaptic currents activated by stimulation of Schaffer collateral-commissural fibres and recorded from interneurones in the CA1 region of hippocampal slices using whole-cell techniques. 2. Interneurones were identified by their location outside the cell layer and their morphology as seen with differential interference contrast (DIC) microscopy and by filling with Lucifer Yellow (LY). 3. The excitatory postsynaptic current (EPSC) had a fast, voltage-insensitive component and a slow component which had a region of negative slope resistance between -70 and -40 mV. The slow voltage-dependent component was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist (DL-2-amino-5-phosphonovalerate (APV) 50 microM) which had little effect on the fast component. Conversely, the fast component was abolished by the non-NMDA receptor antagonist 6-cyano-7-nitoquinoxaline-2,3-dione (CNQX; 10 microM), which had no effect on the slow component. 4. The rise time of the fast component ranged from 1 to 3 ms and the decay time constant ranged from 3 to 15 ms. The rise time of the slow component ranged from 5 to 11 ms and the decay time constant ranged from 50 to 100 ms. 5. It is concluded that although the morphology of the excitatory synapses onto interneurones differs considerably from those onto pyramidal cells, their electrophysiological and pharmacological properties are very similar.

Abstract

We studied with the whole-cell recording techniques, the mechanisms underlying the time course of the slow N-methyl-D-aspartate (NMDA), and fast non-NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) in hippocampal slices. The rising phase of the NMDA receptor-mediated component of the EPSC as well as the decaying phase of the NMDA and non-NMDA component were highly temperature-sensitive, suggesting that neither of these processes is determined by free diffusion of transmitter. Moreover, glutamate uptake blockers enhanced the responses to exogenously applied glutamate, but had no effect on the decay of either the NMDA or non-NMDA components of the EPSCs. On the other hand, open channel blockers known to modify NMDA channel kinetics reduced the EPSC decay time. Thus, the present results support a model in which the rise time and decay of the NMDA component are determined primarily by slow channel kinetics and the decay of the non-NMDA component is due either to channel kinetics or to desensitization.

Abstract

The N-methyl-D-aspartate (NMDA) and non-NMDA classes of glutamate receptor combine in many regions of the central nervous system to form a dual-component excitatory postsynaptic current. Non-NMDA receptors mediate synaptic transmission at the resting potential, whereas NMDA receptors contribute during periods of postsynaptic depolarization and play a role in the generation of long-term synaptic potentiation. To investigate the receptor types underlying excitatory synaptic transmission in the cerebellum, we have recorded excitatory postsynaptic currents (EPSCS), by using whole-cell techniques, from Purkinje cells in adult rat cerebellar slices. Stimulation in the white matter or granule-cell layer resulted in an all-or-none synaptic current as a result of climbing-fibre activation. Stimulation in the molecular layer caused a graded synaptic current, as expected for activation of parallel fibres. When the parallel fibres were stimulated twice at an interval of 40 ms, the second EPSC was facilitated; similar paired-pulse stimulation of the climbing fibre resulted in a depression of the second EPSC. Both parallel-fibre and climbing-fibre responses exhibited linear current-voltage relations. At a holding potential of -40 mV or in the nominal absence of Mg2+ these synaptic responses were unaffected by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV), but were blocked by the non-NMDA receptor antagonist 6-cyano-2,3-dihydro-7-nitroquinoxalinedione (CNQX). NMDA applied to the bath failed to evoke an inward current, whereas aspartate or glutamate induced a substantial current; this current was, however, largely reduced by CNQX, indicating that non-NMDA receptors mediate this response. These results indicate that both types of excitatory input to adult Purkinje cells are mediated exclusively by glutamate receptors of the non-NMDA type, and that these cells entirely lack NMDA receptors.

Abstract

Cone photoreceptors are less sensitive to light and the duration of their photoresponse is shorter than that of rods. In salamander rods and cones, we identified 3 components in membrane currents activated by bright flashes of light: an early receptor current (ERC) resulting from charge displacement within visual pigments, a saturation photocurrent generated by the closure of the cGMP-sensitive channels, and a putative Na-Ca exchanger current. The time courses of both the ERC and the onset of the saturation photocurrent were similar in rods and cones. The putative Na-Ca exchanger current, on the other hand, is 4- to 8-fold faster in cones. The onset of the saturation photocurrent consisted of a delay followed by a fast relaxation with an exponential time course. In both photoreceptor types the delay and the time course of the fast relaxation are dependent on light intensity and reach a limiting value when about 1% of the photopigment is bleached. The limiting value of the delay, about 8 msec, and of the relaxation time constant, about 2 msec, are nearly identical in rods and cones. The near identity of these parameters implies that at least 2 kinetic steps in the activation response of rods and cones are quantitatively similar. These findings suggest that the functional differences between rods and cones may arise from disparities in the processes that restore the components of the phototransduction cascade to their dark level and not from differences in the activation processes.

Abstract

1. The pharmacological and biophysical properties of excitatory synapses in the CA1 region of the hippocampus were studied using patch electrodes and whole-cell recording from thin slices. 2. Excitatory postsynaptic currents (EPSCs) had a fast component whose amplitude was voltage insensitive and a slow component whose amplitude was voltage dependent with a region of negative slope resistance in the range of -70 to -30 mV. 3. The voltage-dependent component was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovalerate (APV; 50 microM), which had no effect on the fast component. Conversely, the fast voltage-insensitive component was abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) which had no effect on the slow component. 4. In Ringer solution with no added Mg2+ the current-voltage relation of the NMDA component was linear over a much larger voltage range than in the presence of 1.3 mM-Mg2+. 5. The NMDA component of the EPSC could be switched off with a hyperpolarizing voltage step at the soma. The kinetics of this switch-off was used to estimate the speed of clamp control of the subsynaptic membrane as well as the electrotonic distance from the soma. The kinetic analysis of the EPSC was restricted to synapses which were judged to be under adequate voltage control. 6. For those synapses that were close to the soma the time constant for decay for the non-NMDA component, which was voltage insensitive, ranged from 4-8 ms. 7. The rise time for the NMDA component was 8-20 ms and the time constant for decay ranged from 60-150 ms. 8. During increased transmitter release with post-tetanic potentiation or application or phorbol esters, both components of the EPSC increased to a similar extent. 9. These experiments provide a detailed description of the dual receptor mechanism operating at hippocampal excitatory synapses. In addition, the experiments provide an electrophysiological method for estimating the electrotonic distance of synaptic inputs.

Abstract

Voltage clamp recordings and noise analysis from pyramidal cells in hippocampal slices indicate that N-methyl-D-aspartate (NMDA) receptors are tonically active. On the basis of the known concentration of glutamate in the extracellular fluid, this tonic action is likely caused by the ambient glutamate level. NMDA receptors are voltage-sensitive, thus background activation of these receptors imparts a regenerative electrical property to pyramidal cells, which facilitates the coupling between dendritic excitatory synaptic input and somatic action potential discharge in these neurons.

Abstract

Using patch-clamp techniques, we recorded single-channel currents from the plasma membrane of the outer segment of isolated light-adapted rods. The channels are potassium-selective and their conductance is about 87 pS. The channels are activated by depolarization and are not sensitive to cytoplasmic calcium, they are exclusively found in rods isolated with the proteolytic enzyme papain, and are not detected in rods isolated by mechanical means. Thus, these channels do not exist in an activatable form in the outer segment plasma membrane under physiological conditions. The channels might be derived from a normally inaccessible structure, such as the disk membrane, or, alternatively, they might be a modified form of a channel that is not active in the intact rod.

EFFECTS OF CYCLIC-GMP ON THE KINETICS OF THE PHOTOCURRENT IN RODS AND IN DETACHED ROD OUTER SEGMENTSJOURNAL OF GENERAL PHYSIOLOGYHestrin, S., Korenbrot, J. I.1987; 90 (4): 527-551

Abstract

We investigated the effects of high concentrations of cytoplasmic cyclic GMP on the photocurrent kinetics and light sensitivity of the tiger salamander rod both in intact cells and in detached outer segments. Photoreceptors were internally perfused with cGMP by applying patch pipettes containing cGMP to the inner or outer segment. Large increases in the concentration of cGMP in the outer segment cytoplasm were achieved only when the patch pipette was applied directly to the outer segment. The dark-current amplitude increased with increasing cGMP concentrations up to approximately 1,400 pA. Internal perfusion with 5.0 mM cGMP introduced a delay of 1-3 s in the photocurrent. The magnitude of the delay was inversely proportional to the light intensity. In addition, the photocurrent time course was slowed down and the light sensitivity, measured 1 s after the flash, was decreased approximately 100-fold when compared with that of the intact cell. The observed effects of cGMP were compared with those predicted by a model that assumes that the initial photocurrent time course is determined by the kinetics of the light-activated phosphodiesterase (PDE) and the cGMP dependence of the light-sensitive channels. At high concentrations of cGMP, the experimental data were similar to those predicted by the model and based on the known biochemical properties of the light-activated PDE and cGMP-activated channels.

THE PROPERTIES AND FUNCTION OF INWARD RECTIFICATION IN ROD PHOTORECEPTORS OF THE TIGER SALAMANDERJOURNAL OF PHYSIOLOGY-LONDONHestrin, S.1987; 390: 319-333

Abstract

1. Rod photoreceptors were isolated from the retinae of tiger salamanders and voltage clamped using the whole-cell patch-clamp technique. 2. Hyperpolarizing the cell to potentials more negative than -50 mV evoked an inward current termed Ih. 3. Ih did not turn on immediately following a hyperpolarizing step but showed a marked delay. The activation time course of Ih could be described by the sum of two exponential components of opposite polarity. 4. The steady-state chord-conductance was half activated at -67 mV. 5. The reversal potential of Ih was close to -30 mV in normal standard salt solution. Increasing the external potassium concentration tenfold shifted the reversal potential by +17 mV. 6. The conductance-voltage relation and the kinetic parameters were not affected by changes in the external potassium concentration. 7. When fully activated, the zero-current conductance underlying Ih depended on the square root of the concentration of external potassium. 8. The permeability ratio PNa/PK depended on the external potassium concentration. It was 0.2 at an external potassium concentration of 2.0 mM and 0.3 at an external potassium concentration of 10.0 mM. The interaction of potassium with Ih suggests that Ih is a multi-ion pore. 9. It is concluded that Ih differs from the inward rectifier that is found in egg cells, frog muscle and heart muscle. 10. The kinetics and voltage sensitivity of Ih suggest that it does not play a role in the dark resting state or in the response to dim flashes of light. Its properties indicate that it may have a major role in the response to bright flashes.

Abstract

We studied, using the patch-clamp technique, the kinetics of single acetylcholine (ACh)-activated channels in a mouse muscle cell line. In the presence of high ACh concentrations we estimated the rate of channel isomerization into the open state (beta) from the dwell time between openings. Also, we obtained estimates for beta under low agonist concentrations by assuming a linear sequential model of channel activation and applying burst analysis. If the linear model is correct, then the two estimates of beta should agree since beta should be independent of ACh concentration. However, the estimates of beta obtained under low ACh concentrations were slower than those obtained independently under high ACh concentrations. The discrepancy in the estimates of beta suggests that the linear model is inadequate, but the discrepancy can be explained if open channels can close through two separate pathways. Two alternative kinetic models that can account for our data are discussed.

Abstract

We have investigated the effect of antibodies from a myasthenic serum on the physiological properties of acetylcholine receptors (AChRs) in myotubes of a mouse muscle cell line, C2. The antibodies in this serum blocked the binding of 125I-alpha-bungarotoxin to the myotubes to an extent of about 50%. The antibodies also inhibited the increase in 22Na influx caused by carbamylcholine (CARB). At a concentration of antibody that blocked about 50% of toxin binding, greater than 80% of the AChR-mediated 22Na influx was blocked. The apparent KD for CARB, estimated from the dose-response curve for 22Na influx, was unaffected. The effect of the antibodies was further examined by patch-clamp recording. In greater than 30% of the patches from antibody-treated cells, no channel activity in response to acetylcholine was seen; in contrast, every patch from control cells showed activity. The channels that were seen after antibody treatment were indistinguishable from those seen in normal cells, both in their single-channel conductance and in the kinetic constants used to describe channel opening and closing. We conclude that the antibodies in this serum inhibit the functional response of AChRs in C2 myotubes to acetylcholine and do so by inactivating individual receptors.

Abstract

In the continued presence of cholinergic ligands, the acetylcholine receptor-channel complex (AChR) in postsynaptic membranes undergoes a sequence of conformational changes. On addition of the ligand, the receptor rapidly changes from a closed channel to an open channel conformation, then slowly changes to a nonconducting state termed desensitization. The lifetime of the open channel conformation and the rate of desensitization are both dependent on the magnitude of the membrane potential, suggesting that the ligand-induced conformational changes in AChR may involve the movement of electrical charges within the membrane. Measurements of charge redistribution in AChR-containing membranes following ligand binding have not been reported. Recently, measurements of changes in the membrane partition coefficient of hydrophobic ions have been used to detect electrostatic changes in both biological and model membranes. We report here that cholinergic ligands induce changes in the partition coefficient of the hydrophobic cation tetraphenylphosphonium (TPP) into AChR-enriched membranes. The extent and time course of these changes in TPP partition coefficient are accounted for in a kinetic model. We conclude that TPP movement is a monitor of a molecular event which may be associated with the slow component of AChR desensitization.

THE INTERACTION OF POTASSIUM WITH THE ACTIVATION OF ANOMALOUS RECTIFICATION IN FROG-MUSCLE MEMBRANEJOURNAL OF PHYSIOLOGY-LONDONHestrin, S.1981; 317 (AUG): 497-508

Abstract

1. Inward rectification of frog muscle membrane was analysed with the Vaseline gap method. 2. Hyperpolarization, under voltage clamp, produced inward potassium currents, which had a component that activated with a time constant, tau K. 3. The activation time constant tau K of the inward potassium currents was voltage dependent. For a given external potassium concentration, the time constant was maximal for potentials near the potassium equilibrium potential, EK. 4. The potassium chord conductance gK, had a sigmoidal voltage dependency, increasing initially e-fold per 11.6 mV of hyperpolarization. 5. When the internal potassium concentration was fixed, raising external potassium induced a shift of the tau K-V and the gK-V relations in the positive direction along the voltage axis. That shift was comparable to the change in EK. 6. No shift of the tau K-V and the gK-V relations was observed when the internal potassium was reduced from 150 to 50 mM. 7. Changes of internal sodium concentration between 5 and 100 mM did not significantly effect the magnitude of inward rectification.